Gas turbine engines generally comprise a compressor for compressing air flowing through the engine, a combustor in which fuel is mixed with the compressed air and ignited to form a high energy gas stream, and a turbine for driving the compressor. One type of gas turbine engine for an aircraft is the turojet in which thrust is provided by the high velocity gas stream exiting the turbine.
A second type of aircraft gas turbine engine is the turbofan in which a fan is mounted forward of the compressor and is driven by a second turbine or power turbine mounted downstream of the first turbine. The fan produces a flow of pressurized air which is split into two portions. The first portion enters an outer bypass duct for bypassing the core engine and the second portion enters the compressor of the core engine. One advantage of the turbofan engine over the turbojet is its ability to move a larger mass of air and thereby increase the thrust output of the engine.
Another feature which may be utilized to increase the thrust output of a gas turbine engine is an augmentor. In an augmented gas turbine engine, an exhaust duct is provided downstream of the turbine(s). Additional fuel is injected into the exhaust duct and is ignited to increase the energy of the gas stream. The gas stream is ejected through an exhaust nozzle to increase the thrust output of the engine.
One type of engine which combines features of the turbofan and augmented engine is a mixed-flow engine where fan airflow is mixed with the core engine gas stream after the turbine but forward of the augmentor. A characteristic of turbofan engines, especially high bypass turbofan engines, is relatively low specific fuel consumption at subsonic speeds. A characteristic of turbojet and relatively low bypass turbofan engines is relatively high specific thrust characteristics at supersonic speeds.
In order to satisfy the need for aircraft which must efficiently operate over a wide range of subsonic and supersonic speeds, so-called variable cycle engines have been developed. Such variable cycle engines are characterized by the capacity to change the bypass ratio of the engine during operation. For example, U.S. Pat. Nos. 4,010,608 -Simmons and 4,175,384 -Wagenknecht et al disclose variable cycle engines. Each variable cycle engine disclosed includes an outer bypass duct and a variable area bypass injector for modulating the flow through the bypass duct thereby varying the engine cycle.
The augmentor in such variable cycle engines is normally located within the exhaust duct from the engine. In order to protect the exhaust duct from the extremely high temperatures associated with the gas flow within the augmentor, a cooling liner may be positioned within the duct so as to form a cooling plenum therebetween. A portion of the bypass flow may then be diverted into this plenum for cooling of the same.
A fundamental problem in the design of an augmentor liner is the pressure differential that may exist between the coolant flow within the plenum and the gas flow inside the liner. The problem becomes particularly acute when the pressure inside the liner suddenly drops. For example, a sudden decrease in the fuel flow to the combustor (throttle chop) will slow the core engine down and reduce the pressure within the augmentor faster than the pressure of the bypass air. Therefore, the design of the liner must provide some means for preventing the inward collapse of the liner.
In the past, various techniques have been proposed to overcome this problem. For example, adequate support such as hangers or couplings may be provided to retain the liner within the duct. However, such solutions add additional complexity and weight and increase the manufacturing cost of the liner. Another means of control is disclosed in U.S. Pat. No. 3,866,417, Velegol, wherein the plenum is divided into a number of individual chambers with flow into each chamber being regulated by flanges which restrict the airflow and regulate the pressure. This system is effective but requires extra structural members to achieve the result. Another solution to the problem is disclosed in U.S. Pat. No. 4,072,008, Kenworthy et al, in which a valve is used to regulate airflow to the augmentor liner. Kenworthy et al discloses an effective means for regulating pressure in the plenum. However, both Velegol and Kenworthy et al require some additional structure to the liner system which results in an increase in cost.